Power and Work Done

• If an 1100 watt microwave is on for three minutes, 198 kilojoules of energy is transferred
• The left lamp is more powerful than the right lamp (60 watts vs. 50 watts)
• Definition of power

  1. Power is the rate at which energy is transferred, shown by the equation power equals energy transferred divided by the time it took to transfer that energy

  2. Power is the rate at which work is done, written as power equals work done over time
• Work done

  A special measure of the energy transferred, used when a force is used to move an object by a certain distance
• Work done examples

  • Pushing a car down the road
  • Transferring electrical energy from a plug socket to a phone
• To push a car down the street using 9 kilojoules of work over 20 seconds, 450 watts of power are used
• Measurement units

  • Power is measured in watts, energy transferred and work done are measured in joules, time is measured in seconds
• Calculating power

  1. Power equals energy transferred divided by time for lamps comparison

  2. Energy transferred equation rearrangement to calculate energy transferred

  3. Calculating power usage for pushing a car down the street